An air-cooled, blow-back operated, automatic weapon experienced erratic firing at temperatures down to -50.degree. F. It was suspected that the lubricant used was unsuitable. The best type of lubricants tried were found to be polytetrafluoroethylene-based, namely, the LSAT as specified by MIL-L-46150 and the T-6 grease of Tribophysics Corporation. Both allow the weapons to function properly at ambient temperature. Neither is designed to meet the stringent requirement of the low temperatures.
Steel surface frictional phenomenon exists in the sliding mechanism of the weapons. Even the polished and chrome-plated surfaces of the cam lever and the bolt are not planar to within 1 micron. When they make rubbing contact with steel surfaces of the rail and receiver, which are also populated with minute roughnesses, the rough peaks on both surfaces undergo shearing, plowing, and welding actions, which account for friction and wear. Given that one of the sliding surfaces has a certain degree of elastic freedom, the motion may be not continuous and may be intermittent in nature, proceeding in a process of stick-slip. The stick is due to the higher static friction between the surfaces, and the slip to the lower kinetic friction during the slipping itself.
When these sliding surfaces are lubricated by applying hydrodynamic lubrication with lubricants such as LSAT, a hydrodynamic film in excess of 1 micron may be formed, which can completely separate surfaces moving relative to each other. Friction is at a practical minimum; that being due only to shearing of the lubricant film, which is a function of the viscosity of the interposed layer. Although viscosity is changed simultaneously by changes in the pressure, temperature, and rate of shear, the most marked effect is due to temperature. Roughly speaking, a lowering of temperature by 18.degree. F. may double the viscosity. Hence, low temperatures in the -50.degree. F. range pose a particular drag to hydrodynamic lubrication.
Boundary lubrication may apply in real-life situations involving high loads, variable speeds (jerky motion), or when a copious, continuous supply of lubricant is unavailable, as at low temperatures, when hydrodynamic lubrication may not be obtained. In that case, the relatively thick lubricant layer breaks down, and the surfaces are separated by boundary films of only molecular dimensions (less than 0.01 micron). The friction is now influenced by the chemical constitution of the lubricant, while the bulk viscosity plays little or no part in the frictional behavior on steel surfaces. At the juncture, boundary lubrication becomes the determining factor in whether seizure will take place or not. The coefficient of friction for unlubricated metal surfaces is about 1.0. The value of surfaces with suitable boundary films is of the order of 0.05-0.10, approaching the best of hydrodynamic lubrication. Teflon is an excellent boundary lubricant due to its low molecular cohesion, and may reduce the coefficient of friction to possibly 0.04.
The LSAT lubricant, covered by MIL-L-46150, contains 25% by weight of Teflon in a dibasic acid ester as the base oil. The torque built up in its presence, maximum 800 g-cm at -65.degree. F., is apparently too high to facilitate the weapon firing at low temperatures. T-6 grease samples by Tribophysics did not function any better under these conditions. When a minimal amount of LSAT was brushed on as a thin film or when the T-6 spray, to which the thixotropic version of T-6 grease is related, was used, the weapon was found to fire normally in the cold. Therefore, the culprit of drag is the base oils which are probably transformed to a relatively stiff matrix when subjected to the extreme cold. This loss in lubricity apparently reduced the counter-recoil velocity and hence the erratic firing of the weapon was observed. Thin-film applications of lubricants are not practical under combat or various environmental conditions, including sand and dust. The need of an extreme climate lubricant suitable for general machine gun usage has not been met.